1. Field of the Invention
The invention relates generally to the field of instruments used to measure electrical conductivity of Earth formations penetrated by a wellbore. More particularly, the invention relates to arrangements of transmitter, receiver and electrode elements for such conductivity measuring instruments.
2. Background Art
Various types of well logging instruments are known in the art for measuring electrical properties of Earth formations penetrated by a wellbore, such as conductivity or its inverse, resistivity. Some such instruments are conveyed into a wellbore at one end of an armored electrical cable and are known as “wireline” instruments. Wireline instruments can also be conveyed into a wellbore using pipe or coiled tubing, but the structure of such instruments is essentially the same as that of the instruments conveyed by armored electrical cable.
Typical wireline conductivity measuring instruments include electrodes and/or wound wire coils disposed on an electrically non-conductive mandrel. Because of the mechanical properties of the materials typically used to make up the mandrel, such as glass fiber reinforced plastic, typical conductivity measuring wireline instruments have one mandrel to house the electrodes and/or wire coils and a separate mandrel to house signal generating and detecting circuitry. One such instrument, which includes longitudinally wound wire coils acting as transmitter and receiver antennas is described in U.S. Pat. No. 4,651,101 issued to Barber et al. and assigned to the assignee of the present invention. Another such instrument which includes electrodes that impact electrical current into the wellbore and surrounding formations and measure resulting voltages is disclosed in U.S. Reissue Pat. No. RE32,564 issued to Scholberg and assigned to the assignee of the present invention.
In any of the foregoing types of instruments, the mandrel that includes the electrodes and/or wire coils may be filled with substantially incompressible liquid such as oil to maintain the integrity of the electrodes and/or wire coils and to prevent crushing of the mandrel under the very high hydrostatic pressures that may be present in a wellbore. The materials used to make the electrically non-conductive mandrel are typically not strong enough to withstand such pressure absent being filled with incompressible liquid. The oil typically must be equalized to the hydrostatic pressure by a compensator such as a piston or elastomer bladder. Such mandrel construction may be expensive and difficult to maintain, and is subject to leakage of the pressure compensation fluid and/or failure of the pressure compensator. Failure of the compensator may cause a catastrophic failure of the mandrel.
Formation resistivity measuring instruments are known in the art for measuring resistivity during the drilling of the wellbore through the subsurface formations. One such instrument is described, for example in U.S. Pat. No. 5,235,285 issued to Clark et al. and assigned to the assignee of the present invention. The instrument described in the '285 patent includes a toroidal antenna disposed on a electrically conductive mandrel. The mandrel is a “drill collar” forming part of a string of drilling tools used to drill through the subsurface formations. Electrical current is passed through the toroidal antenna to induce electrical current to flow, including along the mandrel. One or more electrodes may be disposed in a blade proximate the wellbore wall such that measured voltage at the electrode can provide an indication of the formation resistivity.
Another instrument that may be disposed in a conductive mandrel such as a drill collar is described in U.S. Pat. No. 4,968,940 issued to Clark et al. and assigned to the assignee of the present invention. The instrument disclosed in the '940 patent includes longitudinally wound loop antennas disposed at selected locations on the exterior of a drill collar or similar conductive mandrel. Certain of the loop antennas serve as transmitters and have alternating electrical current typically in a frequency range of 400 kHz to 2 MHz, passed through them to include electromagnetic fields in the formations surrounding the wellbore. Two of the loop antennas serve as receivers. Properties of electrical current induced in the receiver antennas are measured, such as phase shift and amplitude change with respect to the transmitter current. Such measured properties are related to the electrical resistivity of the formation surrounding the wellbore proximate the positions of the loop antennas.